My .02
I would as a GM, wait for the next session, and see how the player works with it.
I would expext him to really hold back power in the next fight, to be unsure of himself.
Perhaps holding of attacking until someone else starts.
As a player, if it was my char
I would do a quick write up, with having nightmares, ask if -2 to all actions do to lack of sleep seem right.
I would play him as timid, and unsure of himself and worried about using his powers.
I killed, a criminal, but I killed.
Am I worthy of of calling my self a Hero, or have I failed.
Should I hang up my cape and retire?
Should I turn myself in?
SHould I flee?
I killed, WHAT right do I have to take a life?
Did he have a wife, childeren, is his mother looking for him.
Have to find out who his family is, have to help them, try to make it up to them.
But How...
Stuff like that
Lee